The high frequency of the sickle allele in some parts of Africa is understood to be a consequence of high malarial endemicity. One corollary of this is that the sickle allele frequency should be declining in populations of African ancestry that are no longer exposed to malaria. We have previously shown that there has been no change in sickle allele frequency in malaria-free Jamaica between two large-scale neonatal screening exercises conducted in 1973-1981 and 1995-2003.

Now I’ve demonstrated that S-allele frequency hasn’t changed in the absence of the “natural-selector”, i.e., malaria.

In fact, S-trait prevalence is reduced in nearly all geographic areas of the African diaspora without malaria. Jamaica has only been malaria-free for about 50 years. The paper suggests their results may be due to a "recent, marked increase" in fitness of SS homozygous individuals.

We found that although model predictions were broadly consistent with observed values in the 1973-1981 cohort, the predicted change in allele frequency between the two cohorts was larger than the observed, nonsignificant, reduction. Close agreement between predicted and observed values was only achieved by simulating a recent, marked increase in HbSS fitness. Thus, the "unexpected" persistence of the sickle allele in Jamaica may reflect the fact that the actual fitness among SS individuals is higher than that previously realized.

So, PaV. Let's assume your intuition is correct and this anomaly supports your contrarian position. So then what comes next? Class? ...hypothesis! ..more data! ... recess, heh... Very good! Class dismissed. ...yeah!... Oh, and next week, we relax our assumptions and allow individuals to mate nonrandomly! There will be lab work and a very stimulating simulation. I'm sure you'll enjoy that topic.

—By the way, PaV, you may want to take your malaria pills when you travel to Jamaica to collect your data. Though Jamaica has been considered non-endemic for 50 years, hundreds of cases of malaria have recently been reported.

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